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"In my view, we're going to be visiting some of the last unexplored worlds in the solar system," said Marc Rayman, Dawn director of system engineering at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif.

Dawn's eight-year mission will carry the 2,685-pound (1,212-kilogram) probe across three billion miles (4.9 billion kilometers) on NASA's first sortie deep into the asteroid belt, a ring of space rocks that circles the Sun between the orbits of Mars and Jupiter.

By visiting the bright, rocky asteroid Vesta and the large, icy Ceres, researchers hope Dawn will shed new light on the formation of planets and solar system's early evolution.

Aside from a wayward ship, which delayed today's launch by 14 minutes when it encroached into the Atlantis Ocean splashdown zone for segments of Dawn's rocket, the liftoff went as planned, NASA launch director Omar Baez said.

Long journey ahead

Dawn is expected to rendezvous and orbit the 330-mile (530-kilometer) wide Vesta between August 2011 and May 2012, then move on to Texas-sized Ceres by February 2015. With its spherical shape and 585-mile (942-kilometer) diameter, Ceres is so large it is also considered a dwarf planet.

"It will be the first mission to journey to, and orbit around, two celestial bodies, and the first to visit a dwarf planet," said Dawn program manager Jim Adams, at NASA's Washington, D.C. headquarters, of the asteroid-bound flight.

Dawn carries an optical camera, gamma ray and neutron detector and a mapping spectrometer to study Vesta and Ceres. Some of those tools will get a trial run during a planned Mars flyby in 2009, researchers said.

To power those instruments, the spacecraft is also equipped with the most powerful solar arrays ever launched into deep space.

With a wingspan of nearly 65 feet (almost 20 meters), or about the distance from the pitcher's mound to home plate on a baseball field, the arrays will generate more than 10 kilowatts near Earth, though that output will decrease as the spacecraft moves further from the Sun.

NASA officials set Dawn's mission cost at $357.5 million excluding the cost of its Delta 2 rocket, according to a September update. In a July briefing, Dawn researchers said the asteroid-bound flight could cost a total of $449 million and incur an extra $25 million in overhead due to launch delays.

Attempts to launch the mission in July were thwarted first by poor weather and rocket glitches, then by difficulties arranging ship and aircraft tracking equipment in time for liftoff. NASA also canceled the mission outright in March 2006, only to reinstate the expedition a few weeks later.

"It has been quite an emotional rollercoaster," Chris Russell, Dawn's principal investigator at the University of California, Los Angeles, said of the mission. "And part of the emotional rollercoaster is the gratitude that we have for all of the people that defended Dawn in those times."

An asteroid trek on ion power

Built by Virginia-based Orbital Sciences, the Dawn spacecraft has been touted as the Prius of space probes because of the uncanny fuel efficiency of its three-engine ion drive.

Dawn carries 937 pounds (425 kilograms) of Xenon gas, to which it gives an electric charge to create ions that are then catapulted out of its engines at nearly 90,000 miles per hour (144,840 kph). Over time, the ion push builds up, and allows Dawn to change its flight path to first rendezvous, then orbit, multiple targets like Vesta and Ceres without requiring massive amounts of conventional rocket fuel.

"The first time I ever heard of ion propulsion was in a 'Star Trek' episode," said Rayman, adding that such engines are also touted to propel the TIE fighters – or Twin Ion Engine - of "Star Wars" fame. "Dawn does the TIE fighter one better because it has three ion engines."

While it will take Dawn four days to go from zero to 60 miles per hour (96 kph), the probe will gradually pick up speed as it fires its ion drive nonstop for the next six years, NASA has said, adding that the mission is the agency's first operational science expedition powered by ion propulsion.

"That would make it the longest powered flight in space history," said Keyur Patel, NASA's Dawn project manager at JPL, just before liftoff.

Each of the three ion engines weighs about 20 pounds (nine kilograms) and is about the size of a basketball.

"From such a little engine you can get this blue beam of rocket exhaust that shoots out at 89,000 miles per hour," Patel said before launch day. "It is a remarkable system."

Libertatem Prius!

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